This invention relates to a circuit arrangement for the driving and independent braking of a vehicle provided with DC traction motors, a DC feeding circuit, and a pulse control.
There is a number of arrangements of electrical accessories of vehicles provided with pulse control; these are, however, mostly provided with a fully working operational mechanical brake, usually a pressure air brake, in addition to the electrical brake.
If there is no operational mechanical brake on the vehicle and the mechanical brake operates only as an emergency brake for final braking, the electrical brake has to be designed so as to engage reliably and to operate under all conditions which may occur in the feeding circuit, even in the case of a breakdown of the feeding voltage, of a short circuit of the trolley system, and in case the feeding circuit receives no power. Similarly, for saving power and for optimum design of the brake resistors, it is required that the fundamental braking be performed by a clean recuperation and that the resistance brake should operate solely as an emergency brake in case the feeding circuit is unable to provide the braking power.
These demanding requirements, however, raise a number of technical problems.
In the first place it is not feasible to use motors with separate excitation in the current connection, although they are otherwise most suitable for pulse control. When the exciting windings are fed solely from the trolley system, these motors cannot be excited in case of a breakdown of the trolley feeding voltage with a simultaneous requirement of braking. If the excitation current is fed over a source of an auxiliary DC voltage, for instance a battery on the vehicle, it is necessary that this battery shall supply all power for excitation required for braking the vehicle if the feeding trolley system breaks down. A battery is not capable of supplying the required power, as its size is limited by the available space on the vehicle, its weight, price and space requirements of the charging source. A combined excitation from a battery on the vehicle and later from the armature of the traction motor can also not be used, as the connection of the battery on the vehicle with the circuit of the armature of the traction motor or from the trolley feeding circuit only via semiconductor elements is not possible. Motors in series connection cannot be used either, because they start their generator operation at low rotating speeds rather unreliably.
The requirement for a clear recuperation within the whole speed range excludes the possibility of application of a dropping resistor and presupposes the maintenance of the armature voltage below the feeding voltage even at highest rotation speeds. This is practically impossible with a current interconnection of two motors in series.
Further, the braking torque of the highest speeds of rotation has to be independent of conditions of the feeding trolley voltage. It is therefore impossible to use a connection with an armature diode, in which case the dependence on the feeding trolley voltage is rather high.